The present invention relates to a plasma processing method and a plasma processing apparatus and, more specifically, to plasma etching used for a dry etching process that ionizes a source gas in a gas phase and processes the surface of a semiconductor material by physical or chemical reaction of highly activated particles of the plasma.
With the advance of miniaturization of semiconductor devices in recent years, there has been a growing tendency to form a wiring layer in multiple layers and to make the device structure three-dimensional. Under these circumstances, the fabrication of an isolation film used to keep wires and devices electrically isolated from one another has come to play an increasingly important role in the device manufacture. Etching of a silicon oxide film, the isolation film, has been done by using perfluorocarbon gas (PFC), such as CF4 and C2F8, and hydrofluorocarbon gas (HFC), such as CH2 and CHF3. This is because a carbon-containing gas is needed to cut off an Si—O bond of the silicon oxide film and generate a volatile compound.
As global environmental concerns are attracting growing attention, PFC and HFC are expected to be subjected to limited use or become difficult to obtain in the future because these gases easily absorb infrared rays, stay in atmosphere for as long as 3000 years and thus contribute greatly to the greenhouse effects on the earth.
The PFC and HFC gas plasmas contain fluorine, fluorocarbon radicals such as CF1, CF2 and CF3, and ions. An etching mechanism of a silicon oxide film operates as follows. These reactive species (e.g., radicals) stick to the surface of the silicon oxide film to be etched. The energy of ions incident on the surface gives rise to a localized quasi-high temperature condition, under which volatile products are formed by chemical reaction. Hence, to obtain good etching characteristics requires controlling the reactive species incident on a sample intended for etching and also controlling the energy and density of ions impinging on the sample. The control of the reactive species and of the density of ions in the plasma has been conducted by a plasma producing system in the etching equipment.
To generate reactive species in a reactor, Japanese Patent Laid-Open No. 74147/1995 for example discloses a method which involves forming the interior of the reactor using a carbon-based material and supplying carbon components into a plasma for etching.
Japanese Patent Laid-Open No. 363021/1992 describes making the reactor using ceramics to prevent degradation in the etching action of reactive species on the sample being etched and also discloses arranging a heater around the periphery of the reactor to alleviate plasma's thermal shocks on the ceramics reactor.
When PFC and HFC gas plasmas are used, fluorocarbon- or carbon-based polymers adhere to the inner wall of the reaction chamber as the etching process of the sample proceeds. A method of removing the adhering polymers is known, which, as described in Japanese Patent Laid-Open No. 62936/1993involves the installation of split, multiple electrodes-isolated from an outer wall of the reaction chamber-on the inner wall of the reaction chamber and the application of a radio frequency (RF) voltage between plasma generating electrodes successively to perform plasma cleaning. Further, Japanese Patent Laid-Open No. 231320/1989, 231321/1989 and 231322/1989 describe plasma cleaning methods which involve applying a voltage to electrodes electrically isolated with respect to the outer wall of the reaction chamber.
If such a conventional plasma cleaning is performed, there still will be particles adhering to the inner wall of the reaction chamber before the next cleaning operation. Because fluorine in the plasma reacts with the adhering layer on the inner wall of the reaction chamber, the fluorine density in the plasma decreases gradually, increasing the ratio of carbon in the plasma. That is, as a growing amount of particles adheres to the inner wall of the reaction chamber, the radical composition changes, causing a time-dependent change in etching characteristic, which poses a serious problem.
Etching equipment can be classified, according to the plasma producing system, into a capacitive coupling type, an ECR (electron cyclotron resonance) type, an ICP (induced coupling) type, and a surface wave excitation type. In the capacitive coupling type etching equipment, a material to be etched is placed on a bottom electrode and two voltage application systems apply differing frequencies and voltage to the upper electrode and the bottom electrode to control the plasma generation and the energy incident on the sample. The structure of this equipment, however, does not allow independent control of plasma generation and incident energy. The control of excited species in this equipment is considered to be performed by carbon or silicon used in the electrodes. However, no parameters on this control are available. Hence, it is necessary to perform three controls, i.e., control of the ion density, control of the energy of ions incident on the material being etched and control of reactive species, by controlling two, upper and lower, power supplies. Therefore, the range of parameters in which satisfactory etching characteristics can be obtained (defined as a process window) is narrow, making it difficult to produce stable etching conditions. The parameters that determine the etching characteristics include, in the plasma generation system, for example, RF power and microwave power applied between the electrodes, gas flow rate, gas pressure and gas composition. In the incident ion energy control, the etching characteristic determining parameters include the waveform and the frequency of the applied voltage and power.
In the plasma generation methods other than the capacitive coupling type, although the plasma generation control and the energy control of ions incident on the sample can be performed independently of each other, the mechanism for controlling the reactive species depends on the plasma generation control. Hence, these plasma generation methods have a drawback of having a narrow process window. In more detail, when a silicon oxide film of SAC (self-aligned contact) is processed in the high density plasma etching equipment, such as an ECR, there is a problem of a tradeoff between etch stopping at the bottom of holes and over-etching into a silicon nitride film. Further, the use of a high density plasma to perform a highly selective etching gives rise to another problem, a micro-loading phenomenon or RIE lag, in which the etching rate decreases as the hole diameters decrease, and an inverted micro-loading phenomenon or inverted RIE lag. Further, when metal films, such as TiN and Al laminated layers, are etched using this equipment, localized abnormal side-etched portions are formed (notching) at the boundary between different materials, such as TiN and Al.
Furthermore, with the method described in Japanese Patent Laid-Open No. 74147/1995it is not possible to control the appropriate amount of excited species, making it difficult to perform an intended etching. The method disclosed in Japanese Patent Laid-Open No. 363021/1992 has a drawback of not being able to generate reactive species in the reactor.